Adaptive transfer case with EM clutch and dog clutch for 4WD lock up

ABSTRACT

An adaptive torque control and distribution system for a motor vehicle having a transfer case, primary driveline and drive wheels and secondary driveline and drive wheels for operation in extreme off road conditions such as sand. The transfer case includes a modulating clutch and a lockup clutch disposed in parallel between its primary (rear) output and secondary (front) output. The system detects abnormally high magnitude and rapidly repeating wheel spin transients which are characteristic of operation in sand or similar highly randomly variable tractive conditions and overrides the on demand torque distribution program for a predetermined time interval. During this predetermined time interval, the transfer case lockup clutch is activated thereby coupling the primary and secondary drivelines and achieving a fifty-fifty torque split therebetween while the modulating clutch is inactive. The system and this operating mode reduces clutch and clutch operator cycling thereby reducing heat generation within the transfer case and increasing clutch and transfer case longevity.

CROSS REFERENCE TO CO-PENDING APPLICATION

This application is a divisional application of Ser. No. 08/570,450,filed Dec. 11, 1995 now U.S. Pat. No. 5,704,444, granted.

BACKGROUND OF THE INVENTION

The invention relates to an adaptive vehicle drive system which shiftsfrom two-wheel drive to four-wheel drive upon sensing certainconditions, and more specifically to an adaptive system which detectshigh magnitude and rapid repetition wheel spin transients and shifts tofour wheel drive operation for a predetermined time period.

The performance advantages of four-wheel vehicle drive systems are wellrecognized. Improved vehicle stability while traversing rain soaked orice or snow covered highways, handling and control on gravel or unevenpavement and simply maintaining traction in off road situations are allreadily acknowledged benefits. Concomitant and less desirable attributesof four-wheel drive systems relate to reduced gas mileage from increaseddrive line friction and increased vehicle weight. Such increased driveline friction occurs in part time four-wheel drive systems whichrotationally couple the front and rear vehicle propshafts. Such vehicleweight increases are particularly pronounced if the system is designedwith a differential between the front and rear drive shafts forfull-time engagement and operation rather than intermittent operationwhen conditions specifically demand it.

Furthermore, while part time four-wheel drive systems which lock thefront and rear propshafts together provide obvious benefits of tractionand stability in straight line driving, the disparity between the groundspeed at the front wheels and the ground speed at the rear wheels duringcornering can itself result in wheel slip and hopping of the vehicle.Thus, allowing the front and rear output shafts of the transfer case tooperate at different speeds during cornering is beneficial.

Many four-wheel drive systems employing diverse control and torquedistribution strategies have been designed and utilized. These variousapproaches are embodied in United States patents.

For example, U.S. Pat. No. 4,417,641 teaches a four-wheel drive systemhaving an electromagnetic clutch and steering sensor. When the steeringwheels are turned greater than a predetermined angle, theelectromagnetic clutch is de-energized, disconnecting two drive wheels.

U.S. Pat. No. 4,718,303 discloses a transfer case having anelectromagnetic ramp clutch which is modulated to adjust torquedistribution in a full time four-wheel drive system. In U.S. Pat. No.4,937,750, a microcomputer compares signals from front and reardriveline speed sensors. If the difference is greater than a certainvalue, a clutch is engaged to interconnect the front and reardrivelines.

U.S. Pat. No. 4,989,686 discloses a four-wheel drive system includingwheel speed detectors. The detectors control a proportional clutch whichdelivers torque to whichever driveline is rotating more slowly. U.S.Pat. No. 5,002,147 discloses a four-wheel drive system which achievestorque splitting between the front and rear axles. The system utilizesfour wheel speed sensors and a steering angle sensor.

In U.S. Pat. No. 5,060,747, a torque distribution system is taught whichincludes both vehicle and front and rear wheel speed sensors. Vehiclespeed data is utilized to adjust the wheel speed difference value andthis adjusted value is utilized to engage a clutch.

U.S. Pat. No. 5,090,510 discloses a four-wheel drive system having adifferential and a hydraulic clutch disposed in parallel between thefront and rear drive shafts.

A nearly universal problem of the foregoing active torque distributionsystems is their operation in extreme off-road conditions, such as sandor mud, where randomly repeated, high magnitude speed differencetransients repeatedly activate and deactivate the torque distributionclutch. Such operation is often justly characterized as unpleasant bythe vehicle operator and occupants because of the abrupt, random andrepeated cycling of the torque distribution clutch which is counter tothe smooth, adaptive torque distribution goal of such systems. A controlstrategy that will recognize operation under such conditions and providea smooth and comfortable operational solution to such conditions istherefore desirable.

SUMMARY OF THE INVENTION

An on demand four-wheel vehicle drive system monitors vehicleperformance and operating conditions and inhibits proportioning torquedistribution operation when certain operating parameters associated withsevere operating conditions such as sand are detected.

The vehicle drive system includes a transfer case having primary andsecondary output shafts driving primary and secondary drivelines, aplurality of speed sensors and a microcontroller. The speed sensorsinclude a primary (rear) and secondary (front) driveline speed sensorand driveline status sensors. The secondary axle may include couplingcomponents such as locking hubs or axle disconnects. The transfer casemay either be a single speed device or may include a planetary gearassembly or similar device providing high and low speed ranges as wellas neutral. The transfer case includes a modulating electromagneticclutch controlled by the microcontroller which selectively transferstorque from the primary output shaft to the secondary output shaft. Thetransfer case also includes a locking clutch which is in mechanicalparallel with the modulating clutch and may be activated to directlycouple the primary output shaft to the secondary output shaft.

Selection of the on demand vehicle drive system both activates thesecondary axle engaging components and may provide a minimum (standby)current to the modulating clutch which establishes a minimum torquetransfer level. When the speed of one of the drive shafts overruns thespeed of the other drive shaft by a predetermined value related to thevehicle speed and the identity of the overrunning shaft, indicating thatwheel slip is present, clutch current is increased to increase clutchengagement and torque transfer to the secondary drive shaft until thespeed difference between the drive shafts and thus wheel slip is reducedbelow the predetermined value. Reduction of the clutch current thenoccurs.

In extreme off road conditions, such as sand, the tractive conditionsmay vary rapidly and dramatically, causing the on demand drive system tocycle randomly, rapidly and repeatedly between substantially fullyengaged and fully disengaged clutch positions. Such operation is lessthan acceptable not only to the driver and passengers but also to thedriveline components which are subjected to accelerated wear. Amicroprocessor reads and computes both average driveline speeddifferences and speed transients which exceed predetermined values basedupon the more slowly turning driveshaft speed. When these values areexceeded, the modulating clutch is fully activated and the lockup clutchis activated after which the modulating clutch is slowly relaxed.Typically the lockup clutch will remain engaged for a five minuteinterval. After the five minute interval, the lockup clutch isdisengaged and system operation returns to normal on demand operation.

The on demand vehicle drive system may be an active full-time system,may be selectively activated by the vehicle operator or may beautomatically activated by driving conditions. The system may beutilized with either primary front wheel or primary rear wheel driveconfigurations.

Thus it is an object of the present invention to provide an on demandsystem which is capable of detecting operation in extreme off roadconditions such as sand.

It is a further object of the present invention to provide a vehicledrive system having both a modulating clutch which functions duringnormal on demand driving conditions and a lockup clutch which engagesduring extreme off road driving conditions.

It is a still further object of the present invention to provide atransfer case having parallel modulating and lockup clutches whichprovide modulating coupling between the primary and secondary output ofthe transfer case as well as providing lockup between such output whiledeactivating the modulating clutch.

It is a still further object of the present invention to provide an ondemand vehicle operating system wherein the sensing of extreme off roadoperating conditions overrides the on demand torque distribution systemand directly couples or locks up the primary and secondary drive linesfor a predetermined period.

It is a still further object of the present invention to provide avehicle drive system which may be utilized in either primary rear wheeldrive or primary front wheel drive vehicle driveline configurations.

It is a still further object of the present invention to provide acontrol system for use with an on demand system to control a transfercase having modulating and lockup clutches disposed in parallel acrossthe primary and secondary outputs.

Further objects and advantages of the present invention will becomeapparent by reference to the following Description of the PreferredEmbodiment and appended drawings wherein like reference numeralsdesignate the same components, elements or features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic plan view of the drive components and sensorsof an on demand vehicle drive system according to the present invention;

FIG. 2 is a full, sectional view of a transfer case and electromagneticclutch assembly in an on demand system according to the presentinvention;

FIG. 3 is an enlarged, fragmentary sectional view of the electromagneticclutch assembly in an on demand vehicle drive system according to thepresent invention;

FIG. 4 is a flat pattern development of a section of one clutch ball andassociated recesses incorporated in the electromagnetic clutch assemblytaken along line 4--4 of FIG. 3;

FIG. 5 is an exploded perspective view of the lockup clutch in atransfer case according to the present invention;

FIG. 6 is a first portion of a software program for detecting extremeoff road driving conditions and controlling a vehicle torque deliverysystem; and

FIG. 7 is a second portion of a software program for detecting extremeoff road driving conditions and controlling a vehicle torque deliverysystem.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Part I--Mechanical Components

Referring now to FIG. 1, an on demand vehicle drive system isillustrated and generally designated by the reference numeral 10. The ondemand system 10 is incorporated into a vehicle having a prime mover,such an internal combustion engine 12, which drives a conventionaltransmission 14 which may either be a manual transmission with a clutchor an automatic transmission. The output of the transmission 14 isoperably coupled to a transfer case assembly 16. In turn, the transfercase is operably coupled to and drives a rear or primary driveline 20having a rear or primary drive shaft 22 which is operably coupled to anddrives a rear or primary differential 24. The primary or reardifferential 24 drives a pair of aligned primary or rear axles 26 whichare coupled to primary or rear tire and wheel assemblies 28.

The transfer case assembly 16 also provides torque to a front orsecondary driveline 30. The secondary driveline 30 includes a front orsecondary drive shaft 32 which in turn drives the front or secondarydifferential 24. The secondary differential 24 operates in conventionalfashion and provide drive torque through a pair of aligned front orsecondary axles 36. A pair of front or secondary tire and wheelassembles 38 are disposed at the front of the vehicle. A pair of lockinghubs 42 are operably disposed between a respective one of the front orsecondary axles 36 and the front tire and wheel assemblies 38. Thelocking hubs 42 may be either remotely operated and thus includeelectrical or pneumatic operators or may be manually activated.Alternatively, front axle disconnects (not illustrated) may be housedwithin the front or secondary differential 34 and the axle disconnectsmay be activated or deactivated to couple or uncouple the secondaryaxles 36 from the output of the secondary differential 34.

The system 10 also include a microcontroller 44 having various programsand subroutines which receive various data from vehicle sensors andprovide control outputs to achieve the design function and goals of thepresent invention which will be more fully described below.

It should be understood that the designations "primary" and "secondary"appearing both above and below refer to drivelines and drivelinecomponents in the system 10 which are primarily and secondarily intendedto propel a vehicle. That is, in the system 10 illustrated, the inventordescribes a vehicle which is commonly referred to as a rear wheel drivevehicle in which the rear tire and wheel assemblies 28 primarily fromboth a time and torque standpoint propel the vehicle. Hence, thesecondary driveline 30 and the front or secondary tire and wheelassemblies 38 typically function intermittently, that is, on an asneeded basis, to provide improved vehicle performance and stability inadverse driving conditions. It should be understood, however, that theoperating components and method described herein are fully and equallyusable and suitable for a vehicle wherein the primary driveline andtires are disposed at the front of the vehicle, that is, a vehiclecommonly referred to as a front wheel drive vehicle, and the secondarydriveline and tires are located toward the rear of the vehicle.

Referring now to FIGS. 2 and 3, it will be appreciated that the transfercase assembly 16 includes a multiple part, typically cast, housing 46having various openings for shafts and fasteners and various mountingsurfaces and grooves for oil seals, bearings, seal retaining rings andother internal components as will be readily appreciated. The transfercase assembly 16 also includes a planetary gear assembly 48 which isdriven by an input shaft 50 rotatably supported within the transfer caseassembly 16. The input shaft 50 is coupled to and driven by the outputof the transmission 14. The input shaft 50 defines a re-entrant bore 52which receives a roller bearing assembly 54. The roller bearing assembly54, in turn, receives and rotatably supports the forward terminus 56 ofa rear (primary) output shaft 58. A gerotor pump 62 is secured about androtates with the output shaft 58, providing lubricating fluid underpressure to a passageway 64 which extends axially within the outputshaft 58 and distributes lubricating and cooling fluid to components ofthe transfer case assembly 16.

In the planetary gear assembly 48, the input shaft 50 defines anenlarged, bell-shaped region 66 having a plurality of external teeth 68which define a sun gear 70. On the inner surface of the bell-shapedregion 66 of the input shaft 50 are a plurality of female splines orgear teeth 72. Axially aligned with the sun gear teeth 68 is a ring gear74 having a plurality of female splines or inwardly directed gear teeth76. A plurality of pinion gears 78, one of which is illustrated in FIGS.2 and 3 are rotatably received upon a like plurality of stub shafts 82which are fixedly mounted within a carrier 84. The carrier 84 includes aplurality of inwardly directed female splines or gear teeth 86 on asurface generally axially adjacent but spaced from the internal searteeth 72 defined by the input shaft 50. The planetary gear assembly 48is more fully described in co-owned U.S. Pat. No. 4,440,042 which isherein incorporated by a reference.

An axially sliding, that is, dog type, clutch 90 is received about theoutput shaft 58. The dog clutch 90 defines an inwardly directedplurality of female splines or gear teeth 92 which are complimentary toand mate with a like plurality of external splines or male gear teeth 94disposed about the periphery of the output shaft 58. The dog clutch 90thus rotates with the output shafts 58 but may slide axially therealong.The dog clutch 90 includes a region of male splines or external gearteeth 96 which are complimentary to the teeth or splines 72 and 86disposed on the input shaft 50 and the planetary gear carrier 84,respectively.

The dog clutch 90 is thus axially translatable between a first, forwardposition wherein the external teeth 96 couple with the gear teeth 72 andprovide direct drive between the input shaft 50 and the output 58 and asecond, rearward position, to the right in FIG. 2 wherein the dog clutch90 engages the gear teeth 86 on the carrier 84 and provides a reducedspeed drive between the input shaft 50 and the output shaft 58 inaccordance with the gear ratio provided by the planetary gear assembly48. A dog clutch 90 may also be moved to a third, neutral positionmid-way between the forward, direct drive position and the rearwardreduced speed drive position. In this middle position, the input shaft50 is disconnected from the output shafts 58 and no torque istransferred therebetween. The position of the dog clutch 90 is commandedby an electric shift control motor 100. The electric shift control motor100 rotates a drive shaft 102. The drive shaft 102 is suitably supportedfor rotation with the housing 46 of the transfer case assembly 16. Theposition of the drive shaft 102 may be monitored and read by an encoderassembly (not illustrated) which provides information about the currentposition of the drive shaft 102 and the dog clutch 90 to themicrocontroller 40.

The drive shaft 102 terminates in and drives a spring assembly 104. Thespring assembly 104 is wrapped about the drive shaft 102 and is alsoengaged by an arm 106 which extends axially from a cylindrical cam 108.The spring assembly 104 functions as a resilient coupling between thedrive shaft 102 and the cylindrical cam 108 to absorb lag between themovement commanded by the drive motor 100 and the driven components sothat the shift motor 100 is allowed to reach its final requestedposition. The spring assembly 104 allows smooth and fast response to arequested repositioning of the dog clutch 90 in situations where thegear teeth 96 of the dog clutch 90 do not instantaneously engage theinternal teeth 72 of the input shaft 50 or the internal gear teeth 86 ofthe carrier 84. When relative rotation of the dog clutch 90 allowsengagement of the aforementioned clutch teeth, potential energy storedin the spring assembly 104 rotates the cylindrical cam 108 to itsrequested position, thus completing the shift.

The cylindrical cam 108 defines a helical track 112 which extendsapproximately 270° about the cam 108. The helical track 112 receives apin or cam follower 114 which is coupled to and translates a forkassembly 116. The fork assembly 116 is supported for bi-directionaltranslation upon a fixed shaft 118 and engages the periphery of the dogclutch 90. Rotation of the shaft 102 axially repositions the camfollower 114 and axially positions the dog clutch 90 in one of the threepositions described above. It will be appreciated that the planetarygear assembly 48 including the mechanism of the dog clutch 90 whichprovides dual range, i.e., high and low speed, capability to thetransfer case assembly 16 is optional and that the vehicle drive system10 is fully functional as a single speed direct drive unit and may beutilized without these components and the dual speed range capabilityprovided thereby.

The transfer case assembly 16 also includes an electromagneticallyactuated disc pack type clutch assembly 120. The clutch assembly 120 isdisposed about the output shaft 58 and includes a circular drive member122 coupled to the output shaft 58 through a splined interconnection124. The circular drive member 122 includes a plurality ofcircumferentially spaced apart recesses 126 in the shape of an obliquesection of a helical torus, as illustrated in FIG. 4. Each of therecesses 126 receives one of a like plurality of load transferring balls128.

A circular driven member 132 is disposed adjacent the circular drivemember 122 and includes a like plurality of opposed recesses 134defining the same shape as the recesses 126. The oblique side walls ofthe recesses 126 and 134 function as ramps or cams and cooperate withthe balls 128 to drive the circular members 122 and 132 apart inresponse to relative rotation therebetween. It will be appreciated thatthe recesses 122 and 134 and the load transferring balls 128 may bereplaced with other analogous mechanical elements which cause axialdisplacement of the circular members 122 and 132 in response to relativerotation therebetween. For example, tapered rollers disposed incomplementarily configured conical helices may be utilized.

The circular driven member 132 extends radially outwardly and is securedto a electromagnetic coil housing 136. The coil housing 136 includes aface 138 which is disposed in opposed relationship with a clutch face140 on an armature 142. The coil housing 136 surrounds anelectromagnetic coil 144 on three sides.

The electromagnetic coil 144 is provided with electrical energypreferably from a pulse width modulation (PWM) control through anelectrical conductor 146. The pulse width modulation scheme increases ordecreases the average current to the electromagnetic coil 144 of theelectromagnetic clutch assembly 120 and thus torque throughput byincreasing or decreasing the on time (duty cycle) of a drive signal. Itwill be appreciated that other modulating control techniques may beutilized to achieve engagement and disengagement of the electromagneticdisc pack type clutch assembly 120.

Providing electrical energy to the electromagnetic coil 144 causesmagnetic attraction of the armature 142 with the coil housing 136. Thismagnetic attraction results in frictional contact of the armature 142 tothe coil housing 136. When the output shaft 58 is turning at a differentspeed than the armature 142 this frictional contact results in africtional torque being transferred from the output shaft 58, throughthe circular drive member 122, through the load transferring balls 128and to the circular driven member 132. The resulting frictional torquecauses the balls 128 to ride up the ramps of the recesses 126 and 134,causing axial displacement of the circular drive member 122. Axialdisplacement of the circular drive member 122 translates a washer 148and an apply plate 149 axially toward a disc pack clutch assembly 150. Acompression spring 152 which may comprise a stack of Belleville washersprovides a restoring force which biases the circular drive member 122toward the circular driven member 132 and returns the load transferringballs 128 to center positions in the circular recesses 126 and 134 toprovide maximum clearance and minimum friction between the components ofthe electromagnetic clutch assembly 120 when it is deactivated.

The disc pack clutch assembly 150 includes a plurality of interleavedfriction plates or discs 154. A first plurality of discs 154A arecoupled by interengaging splines 156 to a clutch hub 158 which iscoupled to the output shaft 58 for rotation therewith. A secondplurality of discs 154B are coupled to an annular housing 160 byinterengaging splines 162 for rotation therewith. An important designconsideration of the recesses 126 and 134 and the balls 128 is that thegeometry of their design and the design of the washer 148, thecompression spring 152 and the clearances in the disc pack assembly 150ensure that the electromagnetic clutch assembly 120 is not self-locking.The electromagnetic clutch assembly 120 must not self-engage but rathermust be capable of controlled, proportional engagement of the clutchdiscs 154 and torque transfer in direct response to the modulatingcontrol input.

The annular housing 160 is disposed for free rotation about the outputshaft 58 and is coupled to a chain drive sprocket 162 by a plurality ofinterengaging splines or lugs and recesses 164. The chain drive sprocket162 is also rotatably disposed on the output shaft 58 and is supportedfreely by a roller or needle bearing assembly 166. When the clutchassembly 120 is engaged, it transfers energy from the output shaft 58 tothe chain drive sprocket 162. A drive chain 168 is received upon thechain drive sprocket 162 and engages and transfers rotational energy toa driven chain sprocket 170. The driven sprocket 170 is coupled to afront (secondary) output shaft 172 of the transfer case assembly 16 byinterengaging splines 174.

The transfer case assembly 16 also includes a first Hall effect sensor180 having an output line 182 which is disposed in proximate, sensingrelationship with a plurality of teeth 184 on a tone wheel 186 which iscoupled to and rotates with the rear (primary) output shaft 58. A secondHall effect sensor 190 has an output line 192 and is disposed inproximate, sensing relationship with a plurality of teeth 194 of a tonewheel 196 disposed adjacent the driven sprocket 170 on the front outputshaft 172. Preferably, the number of teeth 184 on the tone wheel 186 isidentical to the number of teeth 194 on the tone wheel 196 so thatidentical shaft speeds result in the same number of pulses per unit timefrom the Hall effect sensors 180 and 190. This simplifies computationsrelating to shaft speeds and improves the accuracy of all logicdecisions based on such data and computations. As to the actual numberof teeth 184 on the tone wheel 186 and teeth 194 on the tone wheel 196,it may vary from thirty to forty teeth or more or fewer depending uponrotational speeds and sensor construction. The use of thirty-five teethon the tone wheels has provided good results with the Hall effectsensors 180 and 190 and is therefore the presently preferred number ofteeth.

The first and second Hall effect sensors 180 and 190 sense therespective adjacent teeth 184 and 194 and provide a series of pulses inthe lines 182 and 192, respectively, which may be utilized to computethe instantaneous rotational speeds of the rear output shaft 58 and thefront output shaft 172 which, of course, correspond to the rotationalspeeds and the rear drive shaft 22 and the front drive shaft 32,respectively. These rotational speeds may be utilized to infer the speedof the vehicle as well as determine overrunning by either the front orthe rear drive shafts relative to the other which represents wheel spinand thus wheel slip. Hall effect sensors are preferred inasmuch as theyprovide an output signal which alternates between a well defined highand low signal value as the sensed teeth pass.

It will be appreciated that other sensing devices such as, for example,variable reluctance sensors may be utilized. Such sensors do not,however, provide the clean wave form provided by Hall effect sensors,particularly at low shaft speeds, and thus may require extra inputconditioning to provide useable data. It should also be appreciated thatthe Hall effect sensors 180 and 190 and their respective adjacent teeth184 and 194 are preferably located within the housing 46 of the transfercase assembly 16 but may be located at any convenient site along theprimary and secondary drive lines 20 and 30, respectively.

Referring now to FIGS. 2, 3 and 5, the transfer case assembly 16 alsoincludes a locking clutch assembly 200. The locking clutch assembly 200includes a first or internal collar 202 having internal female splinesor gear teeth 204 which engage complimentarily configured male splinesor gear teeth 206 formed in the output shaft 58 along a central portionof its length. The output shaft 58 also includes a step or shoulder 208which limits axial travel of the first collar 202 therealong. A thrustbearing 212 is positioned adjacent the first collar 202. The exterior ofthe first collar 202 includes a first plurality of gear teeth or malesplines 216 which extend fully, axially across its exterior surface.Interleaved with the first set of male splines 216 are a plurality ofaxial, foreshortened channels or grooves 218 which extend only from theright face of the first collar 202 to approximately its midpoint.Concentrically disposed about the first collar 202 is a second orexterior clutch drive collar 222. The clutch drive collar 222 includes apair of peripheral, circumferential ribs 224 which cooperatively definea circumferential groove 226 which, in turn, receives a shift fork 228.

The shift fork 228 is slidably disposed upon the fixed shaft 118 andincludes a cam follower 232 which is received within a cam track 234 inthe cylindrical cam 108.

A compression spring 236 biases the shift fork 228 to the left asillustrated in FIGS. 2 and 3. A compression spring 238 is also receivedwithin the second, exterior clutch collar 222 and provides a biasingforce which tends to drive apart the second clutch collar 222 and athird toothed collar 242. The third or toothed collar 242 includes aplurality of internal gear teeth or female splines 244 which arecomplimentary to and engage the splines 216 and foreshortened grooves218 on the first clutch collar 202. Thus, the third clutch collar 242enjoys limited axial motion on the first clutch collar 202, having itsaxial translation effectively limited by the lengths of the grooves orchannels 218. A snap ring 246 engages a flange 248 on the third clutchcollar 242. The snap ring 246 is received within a channel 252 formed onthe interior of the second clutch collar 222. Finally, the third clutchcollar 242 includes a plurality of gear teeth or male splines 254disposed adjacent and extending axially from the flange 248. The malesplines or gear teeth 254 are complimentary to, aligned with and axiallyengageable with or disengageable from the female splines or gear teeth256 formed in the chain drive sprocket 162.

Part II--Electronic/Software Components

When a typical on demand four wheel drive system, that is, a systemwherein a speed difference between the front and rear wheels or propshafts which exceeds a reference value causes an inter-driveline clutchto selectively couple the two drivelines in accordance with its controlparameters, is driven in deep sand and certain other off roadconditions, the system may cycle randomly, transiently and repeatedlybetween minimum and maximum clutch engagement and driveline coupling. Inmathematical terms, the sum of the speed differences over time greatlyexceeds the sum of speed differences over time of other types ofdriving. As noted above, this cyclic transient operation is unpleasantto the vehicle occupants and is inconsistent with the design goals of ondemand vehicle torque delivery systems. Furthermore, the constantcycling of the clutch creates significant heat within the transfer casewhich can be deleterious to performance and service life expectancies.

In order for a control scheme of an on demand torque system to addressthe challenge of providing appropriately smooth and predictableoperation in a vehicle driving in sand, it is first necessary toestablish a detection protocol to determine when this condition exists.Generally speaking, three categories of vehicle operation and drivingconditions can be defined. They are: a) normal driving conditionsincluding occasional wheel slip and on demand operation, b) situationswhere one or a pair of oversized or undersized tires such as compactspare tires are mounted on the vehicle and c) operation of the vehiclein sand or similar tractive conditions. Each of these three operatingconditions have distinct operating signatures which aid in the detectionof the latter mode. Specifically, determination of a long term averagespeed difference and the number of transients or spikes over such longterm, when properly conditioned, can provide an indication of which oneof the three operating categories represents the current vehicle drivingcondition. In the foregoing statement the designation "long term" meansin the range of from approximately 1 second to 30 seconds and thus issignificantly longer than the operating and active response period ofthe on demand system which is typically measured in milliseconds.

The following Table I sets forth the characteristics of these threeoperating categories.

                  TABLE I                                                         ______________________________________                                                     AVERAGE SPEED                                                                              SPEED TRANSIENTS                                    DRIVING CONDITION                                                                          DIFFERENCE   OR SPIKES                                           ______________________________________                                        Normal       Low          Few                                                 Mis-sized tire                                                                             High         Negligible                                          or compact spare                                                              Sand         High         Many                                                ______________________________________                                    

The foregoing Table I highlights the random cyclic operation of avehicle on demand system in sand which is characterized by rapid cyclingbetween a high speed difference between the front and rear drivelines,followed by a low speed difference after the on demand system engages, arelaxation of the on demand system followed by an abrupt high speeddifference, re-engagement of the on demand system, a low speeddifference, ad infinitum. Accordingly, the software and subroutines ofthe adaptive drive system 10 of the present invention are configured todetect this operating condition. However, it is preferred to determineexperimentally the operating characteristics of each particular vehicleto determined its activity in an extreme operating environment such assand in order to improve the capability of the control system and todifferentiate between extreme conditions which override the on demandfunction or conditions which are not so extreme and allow the on demandfunction to operate conventionally.

Referring now to FIG. 6, a flow chart for a typical microprocessorprogram 300 for the detection of extreme vehicle operating conditionssuch as sand is illustrated. The program 300 is intended to be utilizedwith an on demand system such as that described in co-owned U.S. Pat.No. 5,407,024. The program 300 begins with a start command 302 which maybe commanded by the executive system of the on demand vehicle controlleron a relatively frequent basis such as once every one to five seconds.The program 300 then moves to a decision point 304 which determineswhether the vehicle is operating in on demand torque distribution modeeither by virtue of manual or automatic selection. If it is not, theprogram 300 immediately moves to an end step 306A and returns to theexecutive system. If the vehicle is in on demand torque distributionmode, the program 300 then moves to a second decision point 308 whereina five minute countdown timer is interrogated to determine if it isalready counting. If the response is yes, the program 300 branches to adecision point 310 which will be discussed subsequently.

If the five minute countdown timer is not active, the program 300 movesto a process step 312 which reads the outputs of the Hall effect sensors180 and 190 and computes the average speed difference (delta) betweenthe readings over the last three seconds. The program 300 then moves toa decision point 314 which compares this calculated average speeddifference (delta) with a computed value representing ten percent of theslower of the two drive shafts 22 and 32. If this calculated averagespeed difference (delta) is less than ten percent of the speed of theslower of the two drive shafts 22 and 32, the decision point 314 isexited at NO and the program 300 moves to the end step 306A. If thecalculated average speed difference (delta) for the last three secondsis greater than ten percent of the speed of the slower of the two driveshafts 22 and 32, the decision point 314 is exited at YES and theprogram 300 moves to a process step 316. The process step 316 determinesthe maximum speed difference (delta) between the two drive shafts 22 and32 as sensed by the Hall effect sensors 180 and 190, respectively, overthe last three seconds.

The program 300 then moves to a decision point 318 which compares themaximum speed difference (delta) determined in the process step 316 to avalue representing thirty percent of the slower of the two drive shafts22 and 32. If the maximum speed difference (delta) is less than thirtypercent of the slower of the two drive shafts 22 and 32, the program 300moves to the end step 306A and returns to the system. If, on the otherhand, the maximum speed difference (delta) over the last three secondsis greater than thirty percent of the speed of the slower of the twodrive shafts 22 and 32, the decision point 318 is exited at YES and theprogram 300 moves to a process step 322. The process step 322 commandsan electronic PWM controller (not illustrated) to provide maximumcurrent to the electromagnetic coil 144 such that the disc pack clutchassembly 150 fully engages. When the disc pack clutch assembly 150 isfully engaged, the torque split between the primary driveline 20 and thesecondary driveline 30 is equal, that is, fifty-fifty.

Next, a process step 324 is undertaken which commands the shift motor100 to rotate to translate the shift fork 228, the second clutch collar222 and the third clutch collar 242 to engage the chain drive sprocket162 such that the front or secondary output shaft 172 is directlycoupled to and driven by the primary or rear output shaft 58. At thistime, there is a parallel mechanical connection between the primaryoutput shaft 58 and the secondary output shaft 172 through both the discpack clutch assembly 150 and the locking clutch assembly 200.

The program 300 then moves to a process step 326 which illuminates anindicator light 328 (illustrated in FIG. 1) on the instrument panel ofthe vehicle (both not illustrated) indicating that the vehicle is infour wheel drive operating mode.

Referring now to FIG. 7, the program 300 continues with another processstep 330 which starts a five minute countdown timer. This is the samefive minute timer referenced in decision point 308. The program 300 thenmoves to an additional process step 332 which slowly reduces the controlvoltage or duty cycle to the coil 144 of the electromagnetic clutchassembly 120 over a ten second interval. Next, a decision point 310 isentered which determines whether any shift has been requested by thevehicle operator. The decision point 310 may also be reached fromdecision point 308 if an affirmative response is received to theinterrogation to determine if the five minute countdown timer is active.If no shift has been requested by the vehicle operator, the program 300moves to a decision point 334 which determines whether the speed of thevehicle is greater than thirty-five miles per hour (56 kilometers perhour).

If the vehicle speed is below this threshold, the decision point 334 isexited at NO and the program 300 moves to a decision point 336 whichdetermines whether the five minutes of the five minute countdown timerhave elapsed. If they have not, the program moves to an end step 306Band returns to the system. If the five minutes of the five minutecountdown timer have elapsed, the decision point 336 is exited at YESand the program 300 moves to a process step 338. Also, if in thedecision point 334, it is determined that the vehicle speed is greaterthan the threshold speed, the decision point 334 is exited and theprogram 300 also moves to the process step 338. In the process step 338,the shift motor 100 is commanded to move the shift fork 116 to the leftsuch that the clutch collar 90 directly couples the input shaft 50 tothe primary output shaft 58 and the locking clutch assembly 200 releasesthe coupling between the primary output shaft 58 and the chain drivesprocket 162. Thus the vehicle is returned to two wheel drive high ondemand operation. In the next process step 340, the electromagneticclutch assembly 120 is returned to its normal on demand operation.Finally, the program 300 moves to a process step 342 which clears thefive minute timer, preparatory to a subsequent five minute countdown,cycles to the end step 306B and returns to the executive control system.

Returning to the decision point 310, if a shift has been requested bythe vehicle operator, the program 300 branches to a process step 344.The process step 344 commands the control scheme of the vehicle ondemand system to follow the normal strategy for the requested shift. Ifthe shift requested is appropriate it is carried out. If the shiftrequest is inappropriate it will typically be ignored. Again, thesesteps are under the control of the vehicle on demand system. After theshift request is processed in step 344, the program 300 moves to theprocess step 342, clears the five minute countdown timer and moves toend step 306B.

It should be appreciated that the precise sequence of steps in theforegoing program 300 as well as the numerical values such as the fiveminute countdown timer, the 35 mile per hour threshold, the 10 secondmodulating clutch voltage/engagement reduction period, the three secondperiod for averaging speed, the ten percent average speed differencethreshold and thirty percent transient speed difference threshold arepresented as illustrative and explanatory examples and are not to beconsidered as limiting of the foregoing invention. These values may andtypically will be adjusted over a range of plus or minus fifty percentand perhaps a wider range in order to adapt to the weight, weightdistribution, traction, time constants, torque and torque distributioncharacteristics of a particular vehicle. In fact, the invention mayproperly be considered to be the combination of hardware, that is, atransfer case having both a modulating and lockup clutch and electroniccircuitry and software capable of detecting the rapid, repeated averageand transient speed differences associated with operation of a vehicleon demand drive system in sand and generating control signals whichsequentially engage the modulating clutch and the lockup clutch, allowthe modulating clutch to relax and, after a predetermined period oftime, release the lockup clutch and return to normal, on demand,operation.

The foregoing disclosure is the best mode devised by the inventor forpracticing this invention. It is apparent, however, that devices andmethods incorporating modifications and variations will be obvious toone skilled in the art of on demand vehicle drive systems. Inasmuch asthe foregoing disclosure is intended to enable one skilled in thepertinent art to practice the instant invention, it should not beconstrued to be limited thereby but should be construed to include suchaforementioned obvious variations and be limited only by the spirit andscope of the following claims.

I claim:
 1. A transfer case for providing torque to a first drivelineand a second driveline comprising, in combination,an input for receivingtorque, a first output adapted to receive torque from said input andcoupled to such first driveline and a second output coupled to suchsecond driveline and a clutch assembly operably disposed between saidfirst output and said second output, said clutch assembly having a firstelectromagnetic operator capable of modulatably engaging said first andsaid second outputs in a first state and coupling said first and saidsecond outputs in a second state and a second electromagnetic operatorfor selectively maintaining said first and said second outputs in saidsecond state.
 2. The transfer case of claim 1 wherein said clutchassembly maintains said second state without application of electricalenergy to said first operator.
 3. The transfer case of claim 1 furtherincluding a first controller having a first output driving said firstelectromagnetic operator and a second output driving said secondelectromagnetic operator.
 4. The transfer case of claim 3 wherein saidcontroller sequentially activates said first electromagnetic operatorand said second electromagnetic operator and deactivates said firstelectromagnetic operator.
 5. The transfer case of claim 3 wherein saidcontroller activates said second electromagnetic operator anddeactivates said first operator for a predetermined time period.
 6. Atransfer case for providing drive torque to a first driveline and asecond driveline of a motor vehicle comprising, in combination,a firstoutput adapted to drive said first driveline, a second output adapted todrive said second driveline and a clutch assembly operably disposedbetween said first output and said second output, said clutch assemblyhaving a first electromagnetic operator assembly capable of adjustablyengaging said first output and said second output and fully engagingsaid first and said second outputs, and a second electromagneticoperator assembly for selectively maintaining said first output and saidsecond output in such fully engaged condition.
 7. The transfer case ofclaim 6 wherein said clutch assembly includes a disc pack clutch and adog clutch operably disposed in parallel between said first and saidsecond outputs.
 8. The transfer case of claim 6 further including acontroller for providing electric drive signals to said first electricoperator and said second electric operator.
 9. The transfer case ofclaim 8 wherein said controller terminates said drive signal to saidfirst electromagnetic operator after providing said drive signal to saidsecond electromagnetic operator.
 10. The transfer case of claim 6further including a first speed sensor associated with said firstouptut, a second speed sensor associated with said second output and acontroller for receiving signals from said sensor and providing drivesignals to said first and said second electromagnetic operators.
 11. Anadaptive four-wheel drive system for a motor vehicle for controllablydistributing torque between a primary driveline and a secondarydriveline, wherein the system does not have a geared differential forcoupling the primary driveline and secondary driveline, comprising, incombination,an input member adapted to receive torque, a primary outputarrangement adapted to be driven by the input member and drive theprimary driveline, a secondary output arrangement adapted to drive thesecondary driveline, means for determining the rotational speeddifference between the primary output arrangement and the secondaryoutput arrangement, an electrically powered mover adapted to beactivated in response to the sensed speed difference, a friction clutchassembly activated by the electrically powered mover whereby torque issupplied to the secondary output arrangement so that the primary outputarrangement and the secondary output arrangement are concomitantlydriven; and a controller adapted to modulate electrical power suppliedto the electrically powered mover for a first predetermined period oftime in response to said sensed speed difference, and if a predeterminedsensed speed difference exists at the end of the first predeterminedperiod, substantially simultaneously for a second predetermined periodof time a) engage a locking mechanism to maintain the supply of torqueto the second output arrangement so that the primary output arrangementand secondary output arrangement are concomitantly driven and b) ceasesupplying electrical power to the electrically powered mover.
 12. Thesystem of claim 11 wherein said locking mechanism includes a dog clutch.13. The system of claim 11 wherein said locking mechanism does notpermit the modulation of torque being supplied to the secondary outputarrangement during the second predetermined period of time.
 14. Thesystem of claim 11 wherein said electrically powered mover includes arotatable output member to actuate at least one cam assembly to move thefriction clutch assembly into driving engagement.
 15. A transfer caseassembly for a four-wheel drive motor vehicle comprising, incombination,an input member driving a first output member, a secondoutput member and a clutch assembly operably disposed between said firstoutput member and said second output member, said clutch assembly havinga first electric operator for effecting adjustment of a level of torquetransferred between said first output and said second output and asecond electric operator for effecting a maximum level of torquetransfer between said first output and said second output, and acontroller for first increasing a first drive signal to said firstelectric operator to provide such maximum level of torque transfer,providing a second drive signal to said second electric operator tomaintain such maximum level of torque transfer and reducing said firstdrive signal to said first electric operator.
 16. The transfer caseassembly for a four-wheel drive motor vehicle of claim 15 wherein saidcontroller further includes a timer which effects reduction of saiddrive signal to said second electric operator after a predeterminedperiod of time.
 17. The transfer case assembly for a four-wheel drivemotor vehicle of claim 15 wherein said controller further includes anindicator light and wherein said indicator light is illuminated whensaid drive signal to said second electric operator is present.
 18. Thetransfer cage assembly for a four-wheel drive motor vehicle of claim 15further including a first speed sensor associated with said first outputmember and a second speed sensor associated with said second outputmember, said first and said second speed sensors having outputs providedto said controller.
 19. The transfer case assembly of a four-wheel drivemotor vehicle of claim 18 wherein said controller increases said firstdrive signal to said first electric operator after a period of cyclicoperation to provide such maximum level of torque transfer.
 20. Thetransfer case assembly for a four-wheel drive motor vehicle of claim 19wherein said controller activates said second electric operator anddeactivates said first electric operator.